Morphology

Morphology

These exclusively benthic marine organisms superficially resemble bivalve molluscs in having two hinged shells (valves), yet they have vastly different body plans. While bivalves have two equally sized valves that are oriented laterally (left and right) with respect to the animal’s body, brachiopod shells are oriented up and down as a dorsal valve and (usually) larger ventral valve. Clearly, shell production has converged from separate origins between these two phyla. Another anatomical peculiarity that obviously distinguishes the brachiopods from the bivalves is the presence of a feeding structure called a lophophore. Only two other phyla have lophophores, the ectoprocts (bryozoans) and phoronids. (These three phyla have long been considered a monophyletic group, but recent evidence suggests the “Lophophorates” is paraphyletic; Halanych, 2004.) In brachiopods the lophophore is made up of two arms of ciliated tentacles held out in a rigid ring circling the mouth. Depending on the species, the lophophore may be a simple circle or highly coiled structure, and in most species it has an internal skeletal support (the Brachidium). The lophophore sits inside the water-filled mantle cavity, taking up about two thirds of the space inside the valves and the brachiopod body takes up the posterior third.

Most brachiopods attach themselves to substrates on the ocean floor or bury into soft substrates using a muscular stalk (called a pedicle, but this structure is not homologous between the two brachiopod classes; Kozloff 1990) although some do not attach to anything and remain free-living. To feed, they crack their shells open using didactor muscles (class Articulata, which have valves that articulate with tooth and groove hinges) or hydrostatic pressure (class Inarticulata, which has unhinged valves and no didactor muscles). The cilia on the lophophore tentacles create a feeding current very specific to the shape of the lophophore, which moves particulates along the tentacles. Food particles are then transported to the mouth via branchial food grooves. In general, lophophorates have a blind, U-shaped gut. The Inarticulata has retained an anus and expel solid waste outside of the mantle cavity, however, the larger order Articulata has lost this feature, so secretes fecal pellets back to the mantle cavity though the mouth. Brachiopods can also clear the mantle cavity and lophophore of lodged non-food contaminants and waste products by contracting the adductor muscles sharply to snap the valves closed and create out-flowing currents.

Brachiopods have a contractile heart, but their open circulatory system appears to function primarily in distributing nutrients. Oxygen diffuses mainly across the lophophore and mantle, and is dispersed in the coelomic fluid. Metabolic wastes are discharged from the body into the mantle cavity though metanephridioducts, which are also used to spawn gametes. Brachiopods have a rudimentary nervous system, and a range of sensory organs dependent on where and how they live.